Production of hybrid seed for commercial sale is a large industry. Hybrid plants, grown from hybrid seed, benefit from the heterotic effects of crossing two genetically distinct breeding lines (i.e., hybrid vigor). Typically, the agricultural performance of such hybrid offspring is superior to both parents, for example, in vigor, and yield. This improved performance of hybrid seed varieties, relative to open-pollinated varieties, makes hybrid seed more attractive for farmers to cultivate. To produce hybrid seed that is not contaminated with selfed seed pollination, methods must be employed to ensure cross-pollination and not self-pollination. Historically, pollination control mechanisms have been mechanical, chemical, and/or genetic.
Simple mechanical methods for hybrid seed production typically can only be used if the selected plant species has spatially separate male and female flowers or if the species has separate male and female plants. For example, corn plants have pollen producing male flowers in an inflorescence at the apex of the plant and female flowers in the axils of leaves along the stem. Out-crossing can be insured by mechanical de-tasselling of female plants. Such de-tasselling prevents selfing. However, most major crop plants have both functional male and female organs within the same flower. Accordingly, emasculation of these crop plants is not a simple procedure. It is possible to hand remove the pollen forming organs before pollen is shed, however, this is a labor intensive and expensive form of hybrid seed production.
A second method for the production of hybrid seed is to use chemicals that block or kill viable pollen formation. Such chemicals (gametocides) are used to provide temporary male-sterility. Commercial production of hybrid seed using gametocides is limited by (i) the expense and availability of the chemicals, and (ii) the reliability and length of action of the applications. When plants have extended flowering periods, gametocides are typically not effective because new flowers are produced that are not affected by previous treatment. Repeated application of chemicals is impractical primarily because cost becomes prohibitive.
Other methods of commercial hybrid seed production for field crops rely on a genetic methods of pollination control. In such methods, plants that are used as females (i) fail to make pollen, (ii) fail to shed pollen, or (iii) produce pollen biochemically unable to provide self-fertilization. Plants unable to biochemically self-pollinate are termed self-incompatible plants. Numerous difficulties are associated with the use of self-incompatibilities, including, propagation and availability of the self-incompatible female line, and stability of the self-incompatibility. Self-incompatible systems that can be deactivated are often vulnerable to stressful climatic conditions, where such environmental stresses reduce the effectiveness of the biochemical self-pollination block.
Further methods of commercial hybrid seed production include systems of pollen control based on genetic mechanisms that cause male sterility. Such systems are generally of two types: (i) nuclear or genic male sterility—the failure of pollen formation because of, typically, mutations in one or more nuclear genes; or (ii) cytoplasmic male sterility (CMS)—pollen formation is blocked or aborted as the result of a defect in a cytoplasmic organelle, typically, the mitochondria.
Nuclear sterility can be either dominant or recessive. Dominant sterility can only be used for hybrid seed formation when propagation of the female line is possible, for example, using in vitro clonal propagation. Recessive sterility can be used when sterile and fertile plants are easily discriminated, for example, based on plant phenotypes. Commercial utility of nuclear sterility systems is limited by the expense of clonal propagation and roguing the female rows of self-fertile plants.
Many successful hybridization schemes involve the use of cytoplasmic male sterility. For example, a specific mutation in the mitochondria can, when in a selected nuclear background, lead to the failure of mature pollen formation. Alternately, the nuclear background can compensate for the cytoplasmic mutation resulting in normal pollen formation. A nuclear trait that allows pollen formation in plants having CMS mitochondria, that is, restoration of fertility, is typically called a “restorer gene.” Generally the use of CMS for commercial seed production involves the use of three breeding lines, (i) a male-sterile line (female parent), (ii) a maintainer-line isogenic to the male-sterile line but which contains fully functional mitochondria, and (iii) the male parent line. The male parent line may carry a specific restorer gene(s). In this case the male parent line is usually designated a “restorer line” that imparts fertility to the hybrid seed.
For crops for which seed recovery from the hybrid is unimportant, for example, vegetables, a CMS system can be used without restoration. However, for crops wherein the fruit or seed of the hybrid is the commercial product, then the fertility of the hybrid seed must be restored by specific restorer genes in the male parent or the male-sterile hybrid must be pollinated. Pollination of non-restored hybrids is typically achieved by including, with the hybrid plants, a small percentage of male fertile plants. These male fertile plants effect pollination. In most species, the CMS trait is inherited maternally. This is one restriction of such systems.
A desirable system for hybrid seed production in any crop would be a form of genic male sterility that could be regulated to allow controlled male fertility for the propagation of the female line. The invention described herein provides such a system.